1. Field of the Invention
The present invention relates to a fin structure for stirring a fluid in a heat exchanger and, more particularly, to: a fin structure, which is housed in a heat-transfer tube of a heat-exchanging type cooling apparatus for causing a stirring action to establish turbulent flows or vortex flows in a fluid of a cooled medium or a cooling medium flowing in the heat-transfer tube thereby to enlarge the contact between the heat-transfer tube wall and the fluid, and for making the flow velocity or flow rate of the fluid flowing in the heat-transfer tube uniform thereby to obtain an excellent heat-exchanging function; a heat-transfer tube for a heat exchanger having the fin structure housed therein; and a heat exchanger having the heat-transfer tube assembled therein.
2. Description of Related Art
In recent years, many heat exchangers for fluids of various modes such as liquid-liquid, liquid-gas or gas-gas have been used as not only an EGR cooler for recirculating the exhaust gas of an automobile but also an exhaust gas cooler, a fuel cooler, an oil cooler, an inter cooler, or the like. Various devices have been made in the heat-transfer tube, in which those fluids flow, thereby to efficiently radiate or absorb the heat owned by the fluid. For example, the method, in which the exhaust gas is partially extracted from the exhaust system of a Diesel engine and is returned again to the intake system of the engine and added to the air-fuel mixture, is called the “EGR (Exhaust Gas Recirculation)” to suppress emissions of NOx (nitrogen oxides) thereby to attain many effects to reduce the pump loss and the radiation loss to the cooling liquid, as accompanies the temperature drop of the combustion gas, to increase the specific heat due to the change in the amount/composition of the working gas and to improve the cycle efficiency accordingly. Therefore, the EGR has been widely adopted as the method effective for cleaning the exhaust gas of the Diesel engine or for improving the thermal efficiency.
However, as the EGR gas rises in temperature and increases in flow rate, its thermal action degrades the durability of the EGR valve and may damage the EGR valve early. For this countermeasure against this problem, a water-cooled structure has to be made by providing a cooling system. There is also invited a phenomenon that the charging efficiency is dropped to lower the mileage as the intake temperature rises. In order to avoid this situation, an apparatus has been used to cool the EGR gas with an engine cooling liquid, a car air-conditioning coolant, cooling wind and the like. Of these, there have been proposed many EGR gas cooling apparatus of the gas-liquid heat-exchanging type for cooling the gas or the EGR gas with the engine cooling water. Fins of various modes are housed as means for improving the heat-exchanging performance in the tubes for the EGR gas to flow therein. Of these EGR gas cooling apparatus of the gas-liquid heat-exchanging type, such an EGR gas cooling apparatus of a dual tube heat-exchanging type has been still earnestly demanded as has a simple structure so that it can be easily mounted in a narrow installation space. For example, there have been many dual-tube type heat exchangers including a dual-tube type heat exchanger (as referred to JP-A-11-23181 (pages 1 to 6, FIGS. 1 and 2), for example), in which an outer tube for passing a liquid is arranged around an inner tube for passing a hot EGR gas thereby to perform the heat exchange between the gas and the liquid and in which corrugated metal sheets are inserted as fins in the inner tube, and a dual-tube type heat exchanger (as referred to JP-A-2000-111277 (pages 1 to 12, FIGS. 1 to 12), for example), which includes an inner tube for passing the cooled medium therein, an outer tube space to enclose the outer circumference of the inner tube, and radiating fins arranged in the inner tube and having a thermal stress relaxing function.
According to the dual-tube type heat exchanger having the variously improved fin structure housed therein, the excellent cooling efficiency can be reasonably expected despite of the simple and compact structure. Therefore, many dual-type heat exchangers have already been put into practice as the EGR-gas cooling heat exchanger, the mounting space of which is limited as in a small-sized automobile. Because of the compact structure, the absolute flow rate of the fluid is limited by itself thereby to leave an unsolved problem in the total heat-exchanging amount. In order to solve this problem, the so-called “shell-and-tube type heat exchanger” has to be adopted although it is more or less complicated in structure and has to be large-sized. Various improvements have been done on those heat exchangers. In one example of the shell-and-tube type heat exchanger, a cooling water inlet is attached to one end of the outer circumference of a shell body constituting a cooling jacket, and a nozzle for a cooling water outlet is attached to the other end of the same. A bonnet for introducing a hot EGR gas is integrated with one longitudinal end of the shell body, and a bonnet for discharging the heat-exchanged EGR gas is integrated with the other end of the same. A plurality of flat heat-transfer tubes are attached at a spacing through tube sheets attached to the inner sides of the individual bonnets so that the hot EGR gas flows in the flat heat-transfer tube across the cooling water flowing in the shell body. In addition to the wide heat-transfer area formed by those flat heat-transfer tubes, C-shaped plate fins are fitted on the inner circumferences of the flat heat-transfer tubes thereby to thin the EGR gas flows and to increase the heat transfer area more. Thus, the shell-and-tube type heat exchanger having the excellent heat-exchanging efficiency is disclosed (as referred to JP-A-2002-107091 (pages 1 to 3, FIGS. 1 to 3), for example).
In the aforementioned individual related arts, considerable effects can be expected in that the gas flow is refined to increase the contact area with the corrugated fins or cross fins by housing the fins in the dual-tube type EGR gas cooler, as disclosed in JP-A-11-23181 and JP-A-2000-111277. However, most pipes forming the EGR gas passages have smooth inner circumferences all over the length of the lengthwise direction so that the heat transfer near the centers of the pipes is insufficient. Moreover, the gas flows straight along the EGR gas piping so that the turbulences of the gas flow are insufficient for thinning the boundary layer of the heat-transfer face thereby to make the heat-transferring performance insufficient. In addition, the compact dual-tube structure leaves such a problem unsolved that the absolute value of the calorie to be exchanged is short. In the shell-and-tube type heat exchanger disclosed in JP-A-2002-107091, the plate fins housed in the flat tube are formed straight with respect to the gas flow. As a result, the fluid is so insufficiently stirred that the separation of the stream-lines and the stirring effect of the fluid cannot be said sufficient.
In recent years, moreover, a shell-and-tube type heat exchanger 20, as shown in FIG. 16, is widely adopted not only as the aforementioned EGR gas cooling apparatus but also one example of the heat-exchanging type cooling apparatus including that EGR gas cooling apparatus. In the shell-and-tube type heat exchanger 20, a heat-transfer tube group 23 is formed in a shell 21 for the cooling water to flow therein through tube sheets 25 by a plurality of heat-transfer tubes. The hot fluid, as introduced from a cooled medium inlet g1 formed in a bonnet 22-1, is discharged from a cooled medium outlet g2 disposed in a bonnet 22-2 on the opposite side. In this meanwhile, the hot fluid is heat-exchanged with the cooling water, which flows in the shell 21 through the wall of the heat-transfer tubes forming the heat-transfer tube group 23 in a direction perpendicular to the flow of the cooled medium, so that the hot fluid is cooled to a predetermined temperature. Moreover, individual heat-transfer tubes 23-1 forming the heat-transfer tube group 23 are flattened, as shown in FIGS. 17A to 17C, to enlarge their contact areas. Corrugated plate fins 26, which have a square section and a free shape in the longitudinal direction, are fitted in the flat heat-transfer tube 23-1 thereby to define the passage of the hot fluid or the cooled medium into a plurality of small passages. The plate fins 26 are undulated, as shown in FIG. 17C, to meander the fluid to flow in the small passages thereby to enlarge the heat transfer area. Thus, those fin structures for improving the heat-exchanging efficiency better have been proposed to achieve their individual initial effects. In the heat-transfer tubes having the fin structure formed by subjecting the plate material of a single thin metallic sheet in the flat heat-transfer tube to a special plastic treatment, however, the pressure loss of the fluid in the small passages formed by the fin structure is so low that the fluid to flow between the small passages is not uniformly distributed to make an ununiform distribution in the flow velocity. Moreover, the small passages, which are divided by the plate fins formed of the single metallic thin plate, form the individually independent passages but do not communicate with each other. Therefore, the ununiform distribution of the flow velocity, if once caused, cannot be eliminated to leave such a problem unsolved that the heat-exchanging efficiency is seriously lowered due to that deviation of the flow velocity distribution. Moreover, the ununiformity of the fluid distribution in the divided small passages in the heat-transfer tubes makes it impossible to cool the flowing excess fluid, if any, to the desired temperature range. In case the fluid flow is short, on the other hand, the cooling of the fluid proceeds, but the fluid fails to reach the predetermined flow rate so that the exchanged calorie is resultantly reduced. Even in the aforementioned fin structure improved to raise the heat-exchanging efficiency, difficulties are encountered by the working or mounting method of the fin structure such as the complicated plastic working so that a sufficient performance cannot be attained. The serious problem left unsolved is to make more improvements.